Boron trifluoride is mainly used in industry as a catalyst for many reactions, including polymerization, esterification, alkylation and isomerization, and particularly as a catalyst for the polymerization of olefins. To avoid fluoborate releases, the spent boron trifluoride is generally recovered at the reaction outlet, in the form of solutions of BF.sub.3 hydrate, obtained, on the one hand, by contacting BF.sub.3 with water, and, on the other, by washing with water the organic compounds formed in the reaction catalysed by BF.sub.3 and which, due to the presence of various inorganic and organic impurities, are generally colored to different degrees, with their organic carbon content ranging from a few ppm to a few hundred ppm.
Heretofore these aqueous solutions were either discharged into the river after dilution, or subjected to costly treatments to convert them to products that are harmless to the environment, or to recover and purify the BF.sub.3 for its recycling. Such treatments are described, for example, in patents JP 52/68754, JP 53/96260, JP 56/166987, EP 364 815, U.S. Pat. No. 4,911,758 and U.S. Pat. No. 4,981,578.
Furthermore, fluoboric acid is a valuable commercial product, used particularly as an intermediate in the production of fluoborates, in the electrolytic polishing of aluminium, in surface treatment (pickling) and in the treatment of printed circuits. It is generally marketed in the form of aqueous solutions. The specifications required by end-users impose an HBF.sub.4 content of at least 49% by weight, a boric acid content (H.sub.3 BO.sub.3) of between 0.1 and 8% by weight, and the absence of any color.